This invention relates generally to light-controlled etching of semiconductor material and, more particularly, to an etching method utilizing a laser for controlled etching of a laterally extending undercut in a multilayered semiconductor body.
The varying properties of III-V layered materials have made possible many techniques for the fabrication of novel microstructures. Heretofore, fabrication of such devices have generally involved a multi-step process which sometimes adversely affects the entire surface of the semiconductor body. The laser-controlled aqueous etching of semiconductors is very attractive for this purpose since this process is sensitive to both the electrical and optical properties of the material. It is known from the enclosed papers listed in Table 1, the disclosures of which are hereby incorporated herein by reference, to utilize laser-assisted selective etching of GaAs/AlGaAs systems in the fabrication of various devices. Briefly, Brown et al. use light that is absorbed only in GaAs and not in AlGaAs so as to photo-chemically etch only GaAs on a GaAs/AlGaAs/GaAs substrate sample. An etch stop is generated at the AlGaAs layer with the consequence that these investigators did not observe an undercut. The papers by Deckman et al., Logan et al. and Tijburg et al. disclose the formation of undercuts in multilayered semiconductor structures using dark selective etching, and Kern et al. present an overview of dark chemical etching of bulk materials and on pages 438-462 lists various dark semiconductor etchants. The Podlesnik and Gilgen paper describes the photochemical etching mechanism and discloses devices that have been fabricated utilizing the process. The table on page 116 lists the etchants that were used in the light-controlled etching of the listed semiconductors. The papers not specifically mentioned above disclose other light-generated etching processes including laser-assisted dry etching in a gas ambient and laser-enhanced plasma etching.
In the photochemical etching of bulk semiconductor bodies, the dissolution process is controlled by the flow of carriers across the semiconductor/solution interface. In particular, photogenerated holes initiate an anodic reaction that results in the formation of an oxide which is then soluble in the solution. The transport of these holes to the surface and their resultant spatial distribution therefore determines the morphology of the etched structures. Although the prior art discusses the theory and utility of laser-assisted techniques for controlling liquid-phase etching of bulk semiconductor bodies, there is no apparent recognition that the etching mechanism for bulk material would be operable in layered structures to etch a laterally extending undercut in a buried layer, for example. Applicants have recognized that the band bending at the interface between two layers having different conductivity types, one of which is buried, controls the flow of photogenerated carriers within the structure, resulting in the confinement of these carriers to a desired layer within the sample which changes the morphology of the etched feature relative to what is seen in bulk materials.